Thanks for the reply.
There's a lot of advice out there. The choice of Dacron double-braid looks right. For both splicing, knot-tying, and low stretch.
Ran across this article from Practical Sailor. Check bottom of third paragraph on page 39.
Practical Sailor seems to be a reputable, high-techie pub, so I trust them. Plus they quote Brion Toss who is big on splicing.
Their (both of them) conclusion is that the high-tech synthetic fiber is not good for knots. It either slips or weakens more than it ought to. And more than other kinds of fibers when knotted.
I was just looking for a replacement line for FX tramp that wouldn't stretch, would stay knotted and not slip. Thought about eye splicing some Amsteel for through the tramp and then using Dacron to tie that to eye straps on crossbars. But that looks like too much sugar for a nickel now...
Ropes needed to be rated for grip as well as stretch and strength, I think.